The present invention relates to measuring aggregate bit shift timing error in the read back signal of a magnetic storage system.
In a magnetic storage system, such as a computer disk drive, digital information is magnetically stored upon the surface of a magnetic disk. The digital information is represented by selectively polarizing the magnetic field of consecutive areas across the surface of the rotating magnetic disk. When this information is read back from the storage disk, the magnetic polarization of the medium is sensed and converted into data peaks in an electrical output signal. This reading and writing operation is through a magnetic read/write head which is suspended over the surface of the rotating disk and provides a raw data signal.
The raw data signal is representative of the relative strength of the magnetic flux density present on the magnetic disk. It is highly desirable to provide the highest level of information storage density possible for a given magnetic storage system. Unfortunately, increased storage density leads to significant noise levels in the raw data signal. The raw data signal comprises a series of data "peaks" whose location is used to represent digital information. With high data storage densities, the precise location of a data peak becomes critical because adjacent data peaks are spaced so closely together. If the data peak is shifted slightly in time, incorrect information may be read back which causes a bit shift in the digital data signal. When manufacturing a disk drive system, it is critical that the manufacturer be able to measure bit shift for a given head/disk combination at the desired data storage density. This measure is used to certify the storage system by guaranteeing a maximum error rate at the specified data storage density.
In the disk drive recording industry, it is important to measure where the bits occur within their respective data window. (The data window is a time period used to locate data peaks. A data peak which occurs outside its proper data window produces a readback error.) This measurement is a quality test of the recording system. In a good recording system, all of the bits occur close to the data window center. In contrast, a poor system has bits widely spread early and late with respect to the window center.
Prior art bit shift testers are slow. This is exacerbated because the test must be performed on each disk system manufactured. Prior art bit shift testers are not satisfactory with present-day head specifications because they are very expensive and either slow or inaccurate when used to measure bit shift at the increasingly high recording densities used in modern data storage systems.